2002 Chevy Trailblazer Problems?

When i get to about 1/4 tank of gas my truck will run rough wont go over about 40 mph but no engine light no reduced engine light. i took to to the dealer and they said it was the catalytic converter so i change it.. still doing it so i took it back they said the throttle body so i bought a new one still doing it. keep in mind this only happens at about 1/4 tank and never dies i have replaced the fuel filter catalytic converter throttle body and plugs Can anybody tell me whats going on.... could it be the fuel pump vacuum leak or what ?